Packet detection in point-to-point wireless communication networks

ABSTRACT

Methods, apparatuses, and systems for transmitting information between nodes in a point-to-point wireless communication network is disclosed. One method includes constructing, by a sector of a transmitting node, a packet including data that is to be transmitted to a receiving node in the wireless network, wherein the constructed packet includes a short training field, a channel estimation field, a header field, and a data payload, and transmitting by the sector of the transmitting node, a jamming code before the short training field of the constructed packet, thereby reducing a likelihood that the receiving node will decode a different short training field of an interfering packet before the receiving node decodes the short training field of the constructed packet.

RELATED APPLICATIONS

This patent application claims priority to U.S. Provisional PatentApplication Ser. No. 62/273,993, filed Dec. 31, 2015, further, thispatent application is a CIP (continuation-in-part) of U.S. patentapplication Ser. No. 15/248,297, filed Aug. 26, 2016, and a CIP(continuation-in-part) of U.S. patent application Ser. No. 15/248,378,filed Aug. 26, 2016.

TECHNICAL FIELD

The present disclosure relates generally to wireless communicationnetworks and in particular to methods of minimizing the detection ofinterfering packets in a wireless communication network.

BACKGROUND

Most Internet Protocol (IP) traffic is carried on fiber optic or cablenetworks. This works well when the cable infrastructure is alreadypresent or can be easily installed. However, there are many locationswhere it is either not practical or too expensive to dig up streets orrun cables overhead. To alleviate this problem, wireless networks havebeen proposed to extend the reach of the communication network tolocations that cannot be connected by physical cables. Moreover,wireless networks are generally much easier to reconfigure, e.g. tohandle changes in data communication traffic.

In some wireless networks that have multiple protocol receivers, a datacommunications protocol can identify intended receivers. However, otherprotocols do not provide a capability to target a particular receiver.When no targeting is available, some receivers may miss some packetsbecause they are listening to packets directed at other receivers.

SUMMARY

An embodiment includes a method. The method includes constructing, by asector of a transmitting node, a packet including data that is to betransmitted to a receiving node in the wireless network, wherein theconstructed packet includes a short training field, a channel estimationfield, a header field, and a data payload, and transmitting by thesector of the transmitting node, a jamming code before the shorttraining field of the constructed packet, thereby reducing a likelihoodthat the receiving node will decode a different short training field ofan interfering packet before the receiving node decodes the shorttraining field of the constructed packet.

An embodiment includes a transmitting node. The transmitting nodeincludes a transceiver, an antenna, a processor, and memory. For anembodiment, the memory stores program instructions that are executableby the processor to construct a packet including data that is to betransmitted to a receiving node in the wireless network, wherein theconstructed packet includes a short training field, a channel estimationfield, a header field and a data payload, and control transmission bythe transceiver through the antenna a jamming code before the shorttraining field of the constructed packet, thereby reducing a likelihoodthat the receiving node will decode a different short training field ofan interfering packet before the receiving node decodes the shorttraining field of the constructed packet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrative of a point-to-point wirelesscommunication network in accordance with an embodiment of the presentdisclosure;

FIG. 2 is a block diagram illustrative of a node that receives a desiredpacket and an interfering packet;

FIG. 3 is a block diagram illustrative of a time period during which areceiving node cannot decode a desired packet in the presence of aninterfering packet;

FIG. 4 is a block diagram illustrative of packets transmitted in apoint-to-point wireless network with and without jamming signals inaccordance with an embodiment of the present disclosure.

FIG. 5 shows a transmitting node and a receiving node, according to anembodiment.

FIG. 6 is a flow chart that includes acts of a method for use in awireless communication network, according to an embodiment.

FIGS. 7A, 7B, 7C shows node configurations that can result in packetinterference between links of a wireless network, according to someembodiments.

FIG. 8 shows a desired packet and an interfering packet, according to anembodiment.

FIG. 9 shows a desired packet and an interfering packet, wherein eachpacket includes a different reference sequence, according to anembodiment.

FIG. 10 shows multiple links between transmitting and receiving nodes,and further shows interference between different links, according to anembodiment.

FIG. 11 shows groupings of links, according to an embodiment.

FIG. 12 is a flow chart that includes steps of a method of selectingcodes for groups of links, according to an embodiment.

FIG. 13 is a block diagram of a wireless network, according to anembodiment.

FIG. 14 is a flow chart that includes steps of a method of selectingcodes for sectors of a wireless network, according to an embodiment.

FIG. 15 is a flow chart that includes steps of a method of mitigatingpacket interference by inserting a reference sequence into a preamble ofpackets, according to an embodiment.

FIGS. 16A, 16B, 16C show processes for inserting reference sequencesinto packets, according to embodiments.

FIG. 17 is a flow chart that includes steps of a method of receiving apacket in which a reference sequence is inserted in a preamble of thepacket, according to an embodiment.

FIG. 18 shows test results of packet interference within a wirelessnetwork for different reference code selections, according to anembodiment.

FIG. 19 shows a receiving node that includes received signal gaincontrol, according to an embodiment.

FIG. 20 is a plot that shows GCM (gain control module) selections overtime, according to an embodiment.

FIG. 21 shows a transmitting node, a receiving node, an interferingnode, and a central controller, according to an embodiment.

DETAILED DESCRIPTION

Embodiments introduced herein improve the detection of packets in apoint-to-point wireless communication network. In some embodiments,packets that are transmitted on the network include a training fieldthat a receiver uses to synchronize itself to an incoming packet. Toprevent a receiver from locking on to the training field of aninterfering packet, packets are sent with a jamming signal that precedesthe training field of the packet. The jamming signal prevents a receiverfrom decoding the training field of a packet that was received earlierand which may interfere with the desired packet.

In some embodiments, transmission times in the network are limited to anumber of micro-slots in a time division duplex (TDD) or time divisionmultiplex (TDM) frame. Tightly controlling the transmission times limitsthe maximum time difference of when an interfering and a desired packetcan reach a receiving node.

In some embodiments, one or more portions of a packet, such as the shorttraining field or header field of the packet, are encoded in a mannerthat is specific to an intended receiver. In some embodiments, packetsare transmitted on a frequency that is specific to an intended receiver.

Turning now to the figures, FIG. 1 shows one embodiment of apoint-to-point wireless communication network. The network 100 includesa number of destination nodes (DN) 102 a, 102 b, 102 c, 102 d, etc., anda number of client nodes (CN) 104 a, 104 b, etc. The destination nodestransmit IP packets between themselves and the client nodes. Each nodeincludes one or more antennas, a radio frequency transceiver for sendingand receiving signals and a beamformer (for example, an array ofantennas that are directionally controlled) for selectively changing theeffective transmit and receive directions of the signals transmitted andreceived. The nodes also include one or more processors that executeprogrammed instructions to transmit packets between the nodes or to andfrom a computer communication link. The client nodes transmit andreceive IP packets between themselves and the destination nodes as wellas to a number of end users 106 (including, but not limited to, wirelessenabled devices such as computers, tablets, smart phones, householdappliances, or any other device capable of transmitting and receivingwireless IP data). For at least some embodiments, the destination nodes102 are mounted on utility poles or on buildings and transmitpoint-to-point wireless signals approximately 200-300 meters, dependingon the conditions. The client nodes 104 are generally located inretail/office establishments or in homes to transmit and receive IPpackets to and from the end users. In some embodiments, the IP packetsare sent according to a standardized protocol such as IEEE 802.11ad.However, it will be appreciated that any number of other IP protocols,such as WiMAX 802.16, could be used.

In the network 100, at least one destination node (e.g., node 102 a) iscoupled to a physical cable that carries IP data to and from a computercommunication link 108 (e.g., the Internet or a private communicationlink). IP packets that are destined for an end user 106 are receivedfrom the communication link and are transmitted via one or more routesto the client node 104 b, which is in communication with the end user106. For example, packets may be sent via a route including nodesDN₁->DN₃->CN₁ or via a second route including nodes DN₁->DN₂->CN₁,depending on the radio frequency path conditions that may exist at anytime.

In some embodiments, transmissions are carried on a non-regulated 60 GHzradio frequency spectrum band. At these frequencies, the ability totransmit and receive packets is easily influenced by changingatmospheric conditions (wind, rain, etc.) or by interfering objects(e.g., buses, tree limbs, or other objects passing in and out of theline of sight). Therefore, the best route to complete a communicationlink between a transmitting and a receiving node in the network maychange over time.

In the embodiment shown, a cloud controller computer 110 includes adatabase 112 that stores a list of possible routes that have beendetermined to complete a communication link between the various nodes ofthe network. The cloud controller computer 110 can communicate with eachof the nodes by sending packets that are addressed to the nodes tocontrol the overall operation of the network. In some embodiments, thecloud controller computer 110 periodically updates the list of possibleroutes based on information received from the various nodes in thesystem regarding channel conditions.

To improve the communication path between each of the nodes, to reduceinterference, and to increase the throughput of the network, thedestination and client nodes generally include multiple antennas thatcan be used to control the transmit and receive directions of the nodeby employing beamforming techniques. As will be appreciated by thoseskilled in the art of radio frequency communications, the radiofrequency signals transmitted by each of the antennas can be selectivelytimed by beamforming techniques to direct the main lobe (which cancomprise the bulk of the transmitted signal power) in a desireddirection. Similarly, signals received by the antennas can be delayedand summed using beamforming techniques to change the effectivelistening direction of the receiver. In the embodiment shown in FIG. 1,destination node DN₁ 102 a can beamform its transmitted and receivedsignals in several different directions 103 a, 103 b, 103 c. Similarly,destination node DN₃ 102 c can beamform its transmitted and receivedsignals in directions 103 d and 103 e.

In some embodiments, a node will receive more than one packet at a time.The presence of interfering packets inhibits the ability of a node toaccurately decode a desired packet. This problem is particularly acutein time domain duplex or time domain multiplex point-to-point wirelessnetworks where a common “training field” precedes each packet. In thiscase, the training field does not uniquely identify the intendedreceiver, and it is possible that an unintended receiver will lock on toan interfering packet and miss a desired packet. Therefore, there is aneed for a system that increases the ability of a node to detect adesired packet in the presence of interference.

As discussed above, interfering packets that arrive at a receiving nodeat nearly the same time as a desired packet inhibit the ability of thereceiving node to accurately decode the desired packet. This reducesthroughput on the network because signals have to be sent indicatingthat the desired packet was not received and the missed packet has to beretransmitted. FIG. 2 shows a portion of a communication networkincluding two destination nodes 200, 202 and a client node 204. Each ofthe nodes includes multiple antenna arrays (not shown) so that the nodescan simultaneously transmit or receive packets in different directions.In this example, destination node 200 sends a desired packet 210 in adirection 212 to a first antenna array used by the destination node 202.At nearly the same time, client node 204 sends a packet 214 in adirection 216 to another antenna array used by the destination node 202.In the example shown, the packet 214 sent from client node 204 alsotravels in a path 218 so that it arrives at the node 202 before thedesired packet 210. Because the antenna array on destination node 202 isnot tuned to listen in the direction 218, the packet 214 has a weakersignal power than the signal power of the desired packet 210.

Like other packets transmitted on the network, the interfering packet214 begins with a short training field (STF) 220, is followed by achannel evaluation (CE) field 222 and a header field 224, and ends witha data portion 226 of the packet. The interfering packet 214 is receivedat time t₀ by an antenna array for the node 202 that is listening in thedirection 212. Because the interfering packet 214 arrives at t₀ beforethe desired packet 210 at t₃, the receiver in node 202 attempts to lockon to and decode the interfering packet 214. Due to the presence of thestronger desired packet 210 that arrives later, the receiver is unableto decode the header portion of the interfering packet 214 at time t₁ orthe data portion of the interfering packet at time t₂. However, by thetime the receiver in node 202 determines that it cannot decode theheader field of the interfering packet 214, the short training field ofthe desired packet 210 has already been transmitted. Therefore, thereceiver is also unable to lock on to and decode the desired packet 210.

In some cases, if an interfering packet arrives much earlier than thedesired packet, the receiving node will be able to determine that itcannot decode the interfering packet and still have time to detect anddecode the desired packet. FIG. 3 shows an example of this situation.Here, an interfering packet 240 arrives at the receiving node before adesired packet 250. By the time the receiving node determines that itcannot decode the header portion of the interfering packet, the shorttraining field of the desired packet 250 is just arriving. Even if thereceiving node misses a few samples of the STF of the desired packet250, the receiving node should be able to lock on to the desired packetand decode it. On the other hand, if the interfering packet arrives justbefore the desired packet (e.g., the interfering packet arrives beforethe desired packet by an amount that is less than the duration of theSTF), the receiver will likely be able to lock on to and decode the STFof the stronger packet. Therefore, there is a time window during whichthe interfering packet has the greatest potential to prevent thereceiver from being able to detect and decode a desired packet.

FIG. 4 shows one technique for preventing an interfering packet frominhibiting the detection of a desired packet in accordance with thepresent disclosure. In this embodiment, a desired packet 260 includes ajamming signal code 262 that precedes the short training field of thepacket. An interfering packet 270 also includes a jamming signal code272 that precedes its short training field. The jamming signal code 262of the desired packet 260 has the purpose of preventing a receiving nodefrom being able to lock on to and decode the short training field of apacket that arrives before the short training field of the desiredpacket. In the example shown, the jamming signal code 262 prevents areceiver from locking on to the short training field of the interferingpacket 270. Because the short training field of the interfering packetis not decoded, the receiver will continue to search for a shorttraining field of a received packet and should be able to detect anddecode the desired packet even in the presence of the interferingpacket. In contrast, as is also shown in FIG. 4, if a jamming signal isomitted from a desired packet, the receiver will lock on to the shorttraining field of the interfering packet 290 and the receiver will notbe able to detect and decode the desired packet 280.

In some embodiments, the jamming signal field can comprise a randomsequence or a Golay sequence (sequences with the useful property thattheir out-of-phase aperiodic autocorrelation coefficients sum to zero)that is selected to interfere with the detection of a short trainingfield. In another embodiment, a training transmit/receive (TRN-T/R)field appended to a previous packet can be used instead of, or inaddition to, a short training field. In some embodiments, the jammingsignal has a duration that is about the same length as the shorttraining field.

In accordance with another aspect of the disclosure, the problemscreated by interfering packets can be reduced by tightly controllingwhen packets are transmitted. In some embodiments, a TDD time slot isdivided into multiple micro-slots, each of which is shorter than a TDDtime slot. Packets cannot be transmitted at all times during a time slotbut must be transmitted at a predetermined one of the micro-slots.Because the time period during which packets are transmitted is tightlycontrolled, there is less likelihood that packets will be sent at timesthat cause interference at a receiver. In some embodiments, transmissionschedules for when packets should be transmitted from each node aredetermined by the cloud controller computer 110 shown in FIG. 1. Theschedules from the cloud controller computer are transmitted to eachnode in the network to more tightly control transmissions and lessen thelikelihood of interfering transmissions at a receiving node. Thereceivers in each node subsequently may listen only at times when thedesired signals are scheduled to arrive. If the transmission schedule istightly controlled so that interfering packets do not arrive during thetime window when they have the greatest ability to inhibit the detectionof a desired packet, there is less likelihood that the receiver willlock on to an interfering packet.

In accordance with some embodiments, nodes in the network include areceiver with an automatic gain control circuit (AGC) that operatesaccording to a programmed algorithm to adjust the gain that is appliedto receive signals. In accordance with some embodiments, the AGC of areceiver is lowered during times when a receiver node is not expecting apacket. Therefore, it is less likely that the receiver will detect andlock on to an interfering packet that arrives before a desired packet isexpected.

In accordance with another embodiment, packets are addressed toindividual receiving nodes to lessen the effect that an interferingpacket has at a receiver. In some embodiments, short training fields arecomposed of Golay 128 bit codes followed by negated Golay 128 bit codes.In some embodiments, packets destined for a particular receiver aredefined by a particular sequence of Golay codes (or other trainingcodes). In some embodiments, common training sequences are used until acommunication link is established between nodes. A sequenceidentification is then negotiated for use in further communicationsbetween the two nodes. In some embodiments, the cloud controllercomputer 110 assigns a particular pattern of codes to be used intransmissions to any particular receiver. The codes are forwarded by thecloud controller computer to each of the nodes in the network so thatthe nodes will know how a packet to a particular receiver should beconstructed. In some embodiments, the short training field codes for aparticular receiver are selected in a manner that is unique to thereceiver. In another embodiment, the codes for a header field areuniquely selected for a particular receiver. In this way, a receiverthat receives a packet without the appropriate codes can stop decoding adetected packet and begin listening for another packet on the wirelessnetwork.

If a receiver is able to decode the header field of an interferingpacket, the receiver can be blocked for the entire length of a packet.Therefore, in accordance with another embodiment of the disclosure, theheader field of a packet is scrambled, encrypted or encoded so that itcan be decoded only by the intended receiver. A receiver that detects apacket can attempt to unscramble the header portion. If the headercannot be unscrambled or decoded, then the receiver stops processing thereceived packet and begins searching for new short training fields thatmark the beginning of a new packet.

In some embodiments, the programming of the AGC is adjusted according toa measured or expected link quality. With the AGC reduced, lowerstrength interference signals are less likely to be detected. In yetanother embodiment, a receiver runs both packet detection and packetprocessing operations in parallel. If a new packet with a short trainingfield is detected with a better signal-to-noise ratio or other receptionmetric compared with the packet that is being processed, then thereceiver can eject the packet being processed in favor of the newlydetected packet.

In a manner that is similar to specifically addressing packets toindividual receivers, the transmit frequency on which a receiver detectssignals can be made specific to the particular receiver. In someembodiments, a carrier frequency can be adjusted by a few parts permillion (ppm) in a manner that is specific to a particular receiver.Packets that are to be directed to the particular receiver are sent onthe receiver-specific frequency.

As will be appreciated by those skilled in the art, the presentdisclosure describes a number of techniques that can be used to lessenthe likelihood that an interfering packet will prevent the detection anddecoding of a desired packet. The techniques can be used alone or incombination in transmitting and receiving nodes to increase the abilityof a receiving node to detect desired packets.

In some embodiments, the nodes of the network include a processor, atransceiver for transmitting and receiving wireless signals, and anantenna. At least some of the nodes in the network include multipleantennas in a linear or two-dimensional array that allows the node tochange its transmit and receive directions by beamforming. A memory inthe node stores program instructions that are executable by theprocessor to determine or read directions in which signals should betransmitted from the node. A beamformer in the node is controllable bythe processor to change the direction at which signals are transmittedby the node in order to complete a communication link with another nodein the network.

In some embodiments, each node includes a processor and a memory unitthat stores a routing table with preferred beamforming directions inorder to communicate with other nodes in the network. When a nodereceives a packet to be transmitted to an intended receiving node, theprocessor looks up the beamforming directions and adjusts transmitbeamforming as required to direct the transmissions toward the intendedreceiving node.

In another embodiment, the cloud controller computer stores beamforminginformation to be used by the various nodes in a network. When a packetis to be transmitted between nodes in the network, the beamforminginformation is included in overhead information that is transmitted tothe nodes so that each node will know the preferred beamformingorientation.

An embodiment includes a node for use in a wireless communicationnetwork. The node includes a transceiver, a beamformer (such as, anarray of antennas that are directionally controllable) that isconfigured to beamform signals transmitted by an antenna array, aprocessor; and a memory. For an embodiment, the memory is for storingprogram instructions that are executable by the processor to construct apacket including data that is to be transmitted to another node in thewireless network, wherein the constructed packet includes a shorttraining field, a channel estimation field, a header field and a datapayload, wherein the instructions further include instructions thatcause the processor to encode one or more of the short training field orthe header field in a manner that is specific to an intended receiver.For an embodiment, the transceiver is configured to transmit theconstructed packet on a frequency (carrier frequency) that is specificto the receiving node.

An embodiment includes a node for use in a wireless communicationnetwork. The node includes a transceiver, a beamformer that isconfigured to beamform signals received by an antenna array, aprocessor; and a memory. For an embodiment, the memory is for storingprogram instructions that are executable by the processor to searchincoming signals to detect a short training field of a first receivedpacket, analyze the first packet associated with the detected shorttraining field for data, and continue to search incoming signals todetect a short training field of a second packet concurrently whileanalyzing the first packet, and in an event the short training fieldassociated with the second packet is detected with a better signalstrength than a signal strength associated with the first packet, stopanalyzing the first packet and begin analyzing the second packet.

An embodiment includes a computer-readable storage medium of a node in apoint-to-point wireless communication network storing instructions. Foran embodiment, the instructions include instructions for constructing apacket including data that is to be transmitted to the receiving node inthe wireless network, wherein the constructed packet includes a shorttraining field, a channel estimation field, and a data payload, andinstructions for transmitting a jamming code before the constructedpacket to reduce a likelihood that the receiving node will decode ashort training field of an interfering packet before the receiving nodedecodes the short training field of the constructed packet.

For an embodiment, the instructions include instructions forpre-appending the jamming code to the constructed packet before theconstructed packet is transmitted. For an embodiment, the instructionsinclude instructions for transmitting the jamming code as part of apacket that is transmitted before the constructed packet is transmitted.For an embodiment, the instructions include instructions for encodingthe short training field in the constructed packet to uniquely addressthe receiving node.

An embodiment includes a method of operating a node in a point-to-pointwireless communication network to transmit information to an intendedreceiving node in the wireless network, including constructing a packetincluding data that is to be transmitted to the intended receiving nodein the wireless network, wherein the constructed packet includes a shorttraining field, a channel estimation field, a header field and a datapayload, encoding one or more of the short training field or the headerfield in a manner that is specific to the intended receiving node, andtransmitting to the intended receiving node the constructed packet. Foran embodiment, the method further includes transmitting one or morepackets to the intended receiving node with un-encoded short trainingfields to set up a communication link, and after the communication linkis established, determining an encoding for the short training fieldand/or the header field in a manner specific to the receiving node.

An embodiment includes a method of a node in a point-to-point wirelesscommunication network to transmit information to a receiving node in thewireless network. The method includes constructing a packet includingdata that is to be transmitted to the receiving node in the wirelessnetwork, wherein the constructed packet includes a short training field,a channel estimation field, a header field, and a data payload,determining a frequency offset for packets to be sent to the receivingnode, and transmitting using the frequency offset the constructed packetto the receiving node.

An embodiment includes a method of operating a node in a point-to-pointwireless communication network to transmit information to a receivingnode in the wireless network. The method includes constructing a packetincluding data that is to be transmitted to the receiving node in thewireless network, wherein the constructed packet includes a shorttraining field, a channel estimation field, a header field, and a datapayload, determining a time at which the constructed packet is to besent to the receiving node, wherein the time is one of a number of amicro-slots that together comprise a time slot in a TDD/TDM transmissionsystem, and transmitting during the determined micro-slot, theconstructed packet to the receiving node. For an embodiment, the methodfurther includes encoding one or more of the short training field andthe header field in a manner that is specific to the receiving node. Foran embodiment, the method further includes transmitting one or morepackets to the receiving node with un-encoded short training fields toset up a communication link and once the communication link isestablished, determining an encoding for the short training field and/orthe header field in a manner specific to the receiving node.

An embodiment includes a node for use in a wireless communicationnetwork. The node includes an antenna array, a transceiver coupled toreceive signals from the antenna array, a beamformer that is configuredto beamform signals received by the antenna array, an automatic gaincontrol circuit (AGC) that is configured to adjust a gain applied tosignals received by the antenna array a processor; and a memory. For anembodiment, the memory stores program instructions that are executableby the processor to determine a time slot when signals that are intendedfor the node are expected to be received by the antenna array and tocontrol the AGC circuit to lower the gain applied to signals received bythe antenna at times when signals are not expected to be received. Foran embodiment, the memory further stores instructions that areexecutable by the processor to search incoming signals to detect a shorttraining field of a first received packet, analyze the first packetassociated with the detected short training field for data; and continueto search incoming signals to detect a short training field of a secondpacket as the first packet is being analyzed and in an event the shorttraining field associated with the second packet is detected with abetter signal strength than a signal strength associated with the firstpacket, stop analyzing the first packet and begin analyzing the secondpacket.

FIG. 5 shows a transmitting node 510 and a receiving node 520, accordingto an embodiment. The transmitting node includes a transceiver 512, anantenna 514, a processor 516 and a memory 518. For an embodiment, asector 515 includes the transceiver 512. For at least some embodiments,the transmitting node 510 and/or the receiving node 520 includesmultiple sectors, wherein each sector includes a radio (transceiver)).For an embodiment, the memory 518 stores program instructions, whereinthe program instructions are executable by the processor 516 toconstruct a packet including data that is to be transmitted to thereceiving node 520 in the wireless network, wherein the constructedpacket includes a short training field, a channel estimation field, aheader field and a data payload. Further, the program instructions areexecutable by the processor 516 to control transmission by thetransceiver 512 through the antenna 514 a jamming code before the shorttraining field of the constructed packet, thereby reducing a likelihoodthat the receiving node will decode a different short training field ofan interfering packet (such as, from interfering node 530) before thereceiving node 520 decodes the short training field of the constructedpacket.

For an embodiment, the constructed packet is to be transmitted to thereceiving node 520 over a specific link between the transmitting nodeand the receiving node, and wherein constructing the packet fortransmission over the specific wireless link includes the programinstructions executing the processor to identify a reference sequencebased on the specific wireless link, insert the reference sequence intoat least a portion of a preamble of the packet, and control transmissionof the constructed packet through the over the specific wireless link.

For at least some embodiments, the processor identifies the referencesequence by accessing the reference sequence from storage. That is, foran embodiment, references codes are predetermined for each specific linkbetween the transmitting node 510 and other nodes. The reference codesare retrieved and used based upon the specific link the constructedpacket is to be transmitted over.

For at least some embodiments, identifying the reference sequence basedon the specific wireless link includes the program instruction executingthe processor to retrieve the reference sequence. Before the referencesequence is retrieved, interference between the specific link and atleast one other link of the wireless network is characterized, andwherein the reference sequence is assigned to the specific link, andanother sequence is assigned to the at least one other link of thewireless network based on the characterizing of the interference. Theassigned sequences can be stored for future retrieval.

For at least some embodiments, identifying the reference sequence basedon the specific wireless link includes the program instruction executingthe processor to retrieve the reference sequence. The reference sequencecan be retrieved from the local memory 518, or from external storage.Before the reference sequence is retrieved, for at least someembodiments, a subset of codes from available codes is selected.Further, links of the wireless network are grouped into a plurality ofgroups based on connectivity of the links between transmitting nodes ofnodes of the wireless network, interference between at least one link ofa first group of the plurality of groups and at least one link of asecond group of the plurality of groups is characterized, wherein atleast one of the first group or the second group includes the specificwireless link. Further, at least one code of the subset of codes isassigned to the first group and at least one other code of the subset ofcodes to the second group based on the characterizing of theinterference. Further, the transmitting node 510 is configured(configuration can include storing the reference sequence in theexternal storage, in the memory 518, or making the reference sequenceavailable to the transmitting node by some other available means) withthe references sequence, wherein the reference sequence includes one ofthe subset of codes of the first group or one of the subset of codes ofthe second group based on which of the first group or the second groupincludes the specific wireless link.

FIG. 6 is a flow chart that includes acts of a method for use in awireless communication network, according to an embodiment. A first act610 includes constructing, by a sector of a transmitting node, a packetincluding data that is to be transmitted to a receiving node in thewireless network, wherein the constructed packet includes a shorttraining field, a channel estimation field, a header field, and a datapayload. A second act 620 includes transmitting by the sector of thetransmitting node, a jamming code before the short training field of theconstructed packet, thereby reducing a likelihood that the receivingnode will decode a different short training field of an interferingpacket before the receiving node decodes the short training field of theconstructed packet. An embodiment includes pre-appending the jammingcode to the constructed packet. An embodiment includes post-appendingthe jamming code to a packet that is transmitted before the constructedpacket is transmitted. For an embodiment, transmitting the jamming codebefore the short training field includes transmitting the jamming codeimmediately before the short training field. For an embodiment, theconstructed packet includes the jamming signal code 262 that precedesthe short training field of the constructed packet. As previouslydescribed, for an embodiment, the interfering packet 270 also includes ajamming signal code 272 that precedes its short training field. Thejamming signal code 262 of the desired packet 260 has the purpose ofpreventing a receiving node from being able to lock on to and decode theshort training field of a packet that arrives before the short trainingfield of the desired packet.

As previously described, in some embodiments, the jamming signal fieldcan comprise a random sequence or a Golay sequence (sequences with theuseful property that their out-of-phase aperiodic autocorrelationcoefficients sum to zero) that is selected to interfere with thedetection of a short training field.

An embodiment includes coding the short training field of theconstructed packet to uniquely address the receiving node. An embodimentincludes coding the header field of the constructed packet to uniquelyaddress the receiving node. For an embodiment, the coding includesidentifying a reference sequence based on a specific wireless linkbetween the sector of the transmitting and the receiving node.

For an embodiment, the constructed packet is to be transmitted to thereceiving node over a specific link between the transmitting node andthe receiving node. Further, constructing the packet for transmissionover the specific wireless link includes identifying a referencesequence based on the specific wireless link, inserting the referencesequence into at least a portion of a preamble of the packet, andtransmitting, by the sector, the configured packet over the specificwireless link. For an embodiment, the jamming code is the referencesequence.

For an embodiment, the sector of the transmitting node 510 performs theidentifying of the reference sequence. For an embodiment, centralcontroller 550 performs the identifying of the reference sequence, andthe central controller provides the reference sequence to the sector.

For an embodiment, inserting the reference sequence into the at leastthe portion of the preamble includes pre-appending the referencesequence to the at least the portion of the preamble.

For an embodiment, the reference sequence includes a complementarysequence. For an embodiment, the complementary sequence includes a Golaysequence. An embodiment includes repeating the reference sequenceinserted into the preamble. For an embodiment, a phase of at least aportion of the repeating reference sequence changes within the preamble.

For an embodiment, identifying the reference sequence based on thespecific wireless link includes characterizing interference between thespecific link and at least one other link of the wireless network, andassigning the reference sequence to the specific link, and anothersequence to the at least one other link of the wireless network based onthe characterizing of the interference.

For an embodiment, identifying the reference sequence based on thespecific wireless link, includes selecting a subset of codes fromavailable codes, grouping links of the wireless network into a pluralityof groups based on connectivity of the links between sectors of nodes ofthe wireless network, characterizing interference between at least onelink of a first group of the plurality of groups and at least one linkof a second group of the plurality of groups, wherein at least one ofthe first group or the second group includes the specific wireless link,assigning at least one code of the subset of codes to the first groupand at least one other code of the subset of codes to the second groupbased on the characterizing of the interference, and configuring thesector with the references sequence, wherein the reference sequencecomprises one of the subset of codes of the first group or one of thesubset of codes of the second group based on which of the first group orthe second group includes the specific wireless link.

The embodiments described include methods, apparatuses, and systems formitigating packet interference between sectors of a wireless network. Atleast some embodiments include receiving, by a sector, data to betransmitted over a specific wireless link of a wireless network. Apacket for transmission over the specific wireless link is configured,wherein the packet includes a preamble, and the data. Further, theconfigured packet is transmitted, by the sector, over the specificwireless link. For at least some embodiments, configuring the packetincludes identifying a reference sequence based on the specific wirelesslink, and inserting the reference sequence into at least a portion ofthe preamble.

FIGS. 7A, 7B, 7C shows node configurations of a wireless network thatcan result in packet interference between links of the wireless network,according to some embodiments. FIG. 7A shows wireless nodes 710, 712,714, 716 of the wireless network. Wireless node 710 may be wirelesslycommunicating with node 712. However, due to proximity or other factors,this wireless communication may cause interference with reception atnode 716. That is, node 716 may be receiving wireless communication fromnode 714, but the wireless communication between node 710 and node 712may interfere with the reception of the wireless communication by node716.

FIG. 7B shows node 722 wirelessly communicating with client node 720.Further, client node 726 is communicating with node 724. However, thewireless communication between node 722 and client node 720 mayinterfere with the communication between client node 726 and node 724.

FIG. 7C shows a node 730 wirelessly communicating with a first sector ofnode 734, and node 732 wirelessly communicating with a second sector ofnode 734. For at least some embodiments, a node includes multiplesectors, wherein each sector includes at least a transceiver. However,the communication signal emanating from node 730 may reflect off of areflector 740 and be redirected to interfere with the wirelesscommunication between the node 732 and the second sector of node 734.

As shown, due to the large number of proximate wireless links,multipoint, multi-hop wireless networks are susceptible to interferencebetween links of the wireless network. That is, wireless communicationof one link between nodes of the wireless network can interfere with thewireless communication of one or more other link between other nodes ofthe wireless network.

FIG. 8 shows a desired packet 810 and an interfering packet 820,according to an embodiment. As shown, a receiver of a node of thewireless network can receive an interfering packet 820 before receivinga desired packet 810. The desired packet is an intended or desiredpacket of wireless communication over a link between the node and atransmitting node. However, due to the existence of other nodes of thewireless network, the node may also receive the interfering packet 820 atime t before receiving the desired packet 810.

The interfering packet 820 is generated by another node of the wirelessnetwork. Therefore, the interfering packet may be commonly constructed.For example, for an embodiment, one or more packets of the wirelessnetwork include a preamble that includes a short training field (STF)and a channel estimation (CE) field. Further, for an embodiment, the oneor more packets further include a header and data.

Upon receiving the interfering packet 820 (also referred to as earlyweak interference) the receiver of the node may attempt to lock onto theearly weak interference packet 820. That is, the early weak interferencepacket 820 may be similarly constructed as the desired packet 810, butthe early weak interference packet 820 is received by the node a time tbefore receiving the desired packet 810. Therefore, the receiver of thenode may misinterpret the early weak interference packet 820 as thedesired packet 810, and attempt to lock onto the packet in order toreceive and decode the packet. That is, for at least some embodiments, areceiving sector of a node uses the STF to detect the presence of apacket, and thereafter starts the remainder of the packet acquisitionprocess. If the node inadvertently uses the STF of the weak interferencesignal to detect the presence of the packet, the node may start theremainder of the acquisition process. However, the early weakinterference packet 820 is not the desired packet, and while thereceiver of the node is attempting to lock onto and decode the earlyweak interference packet 820, the desired packet may be missed. Clearlythis is undesirable.

FIG. 9 shows a desired packet 910 and an interfering packet 920, whereineach packet includes a different reference sequence, according to anembodiment. For at least some embodiments, a transmitting sector of atransmitting node inserts a reference sequence into at least a portionof the preamble of a packet to be transmitted. For at least someembodiments, the reference sequence is a specific reference sequencethat is associated with a specific link. That is, for example, thespecific link between the transmitting node and the node receiving thedesired packet 910 has an associated reference sequence that thetransmitting node inserts into at least a portion of the preamble of thedesired packet 910. The node receiving the desired packet is able toidentify the specific reference sequence for the specific link betweenthe transmitter and the node, and the node uses this specific referencesequence to correlate the at least a portion of the preamble with thereference sequence that is selected based on the specific wireless link.The receiving nodes may start the remainder of the acquisition process.

For an embodiment, the transmitter of the weak signal interferencepacket 920 also inserts a different reference sequence into the preambleof the interfering packet 920. The reference sequences of the packets ofinterfering links are selected to be uncorrelated with the referencesequence of the desired packet of the specific link. Therefore, the nodewill not “lock on to” (that is, begin and continue processing) theundesired interfering packet 920 because the reference sequence of theinterfering packet 920 is uncorrelated with the specific referencesequence associated with the specific link. Further, the receiving nodewill not begin the packet acquisition process when receiving theinterfering packet 920, but rather, start the packet acquisition processupon receiving the desired packet 910.

FIG. 10 shows multiple links between transmitting and receiving nodes,and further shows interference between different links, according to anembodiment. At least some embodiments include identifying links of thewireless network that interfere with other wireless links of thewireless network. At least some embodiments include identifying groupsof links of the wireless network that interfere with other groups ofwireless links of the wireless network.

As shown, a link j is formed between a transmitting sector 1010 and areceiving sector 1020. Further, a link i is formed between atransmitting sector 1011 and a receiving sector 1021. Further, a link kis formed between a transmitting sector 1012 and a receiving sector1022.

Due to the proximity and relative physical locations of each of thesectors and the links formed between the sectors, at least some of thelinks will cause interference within other links. The interference canbe further influences by reflectors that vary the direction of travel ofthe wireless communication of the links, and by variations in thedirectivity of beams formed by multiple antennas of at least someembodiments of the sectors.

For example, the link j may cause at least some interference with thelink i which can be represented by an indicator of the interferencepower R_(ji). Further, the link i may cause at least some interferencewith the link k which can be represented by an indicator of theinterference power R_(ik).

For at least some embodiment, the interfering packets of theseinterfering link have reference sequences inserted into at least aportion of the preambles of the interfering packets to prevent receiversof victim links (wireless links that suffer from interference due to theinterfering links) from attempting to lock onto the interfering packetsof the interfering links.

Grouping of Links

FIG. 11 shows groupings of links, according to an embodiment. Aspreviously described, the effects of packet interference can bemitigated by inserting a different reference sequence into the preambleof the packet communicated through each different link. That is, for anembodiment, each group includes a single link. However, the number ofpossible reference sequences can be limited, and processing overhead isincreased as the number of different reference sequences is increased.Accordingly, for an embodiment, the links are grouped. Once the linksare grouped, a group that is determined to include one or more linksthat interfere with one or more links of another group is assigned acode (reference sequence) that is different, and uncorrelated with thereference sequence of the other group. Sectors associated with the oneof more links of a group are assigned the reference sequence selectedfor the group. The sectors then use the assigned code for packetscommunication over the specific one or more links of the associatedgroup. For an embodiment, a single code (reference sequence) is used forboth transmission and reception of packets by a sector. For anembodiment, a first code (reference sequence) is used for transmissionof packets by a sector, and a second code (reference sequence) is usedfor reception of packets by the sector.

For an embodiment, grouping links of a wireless network into a pluralityof groups is based on a connectivity of the links between sectors of thewireless network.

FIG. 5 shows groupings of links, according to an embodiment. A firstgroup (Group1) includes links formed between sectors 1110, 1111, 1112.The sectors are each a part of different nodes of the wireless networkand include two-way (transmit and receive) communication.

For an embodiment, the links are grouped by the wireless connectivitybetween communicating sectors. For example, sectors 1110, 1111, 1112 aredirectly connected (that is, there is a single wireless hop between anytwo of these sectors), and form a first group (Group1).

A second group (Group2) includes sectors 1113, 1114, 1117 due to thedirect connectivity of these sectors. A third group (Group3) includessectors 1115, 1116, 1118 due to the direct connectivity of thesesectors.

As will be described, if one or more of the links of one group interferewith one or more links of another group, the two groups are assigneddifferent codes (reference sequences) to mitigate interferences betweenlinks of the two groups.

FIG. 12 is a flow chart that includes steps of a method of selectingcodes for groups of links, according to an embodiment. A first step 1210includes selecting a plurality of available codes, which can includeselecting a subset of codes from available codes. That is, a set numberof say 128 codes may be available. However, for an embodiment, thenumber of codes may be limited to ensure a level of correlation betweeneach of the codes that is below a desired threshold. The less correlatedthe codes, the less likely that a sector assigned one code will beinadvertently received and attempt to decode a packet having anothercode. For an embodiment, the codes include a complex valued constituentbase sequence. For an embodiment, the codes include complementarysequences or codes. For an embodiment, the codes include Golay codes.For an embodiment, the codes include a pseudo random sequences or codes.For an embodiment, the codes include a random complex sequence.

A second step 1220 includes grouping links of a wireless network into aplurality of groups based on connectivity of the links between sectorsof the wireless network. The connectivity of links can be used to selectthe different groups of links. A link includes a transmitting sector anda receiving sector. For an embodiment, a link includes a pair-wiseconnection between two sectors.

A third step 1230 includes characterizing interference between at leastone link of a first group of the plurality of groups and at least onelink of a second group of the plurality of groups. For an embodiment,the characterizing interference between at least one link of a firstgroup of the plurality of groups and at least one link of a second groupof the plurality of groups includes measuring the interference. Forexample, one sector associated with at least one of the links of onegroup can measure a level of interference received from another sectorassociated with at least one of the links of one other group. Eachsector includes a transceiver which receives wireless signals. The levelof interference of the received wireless signals can be measured. For anembodiment, the measured level of interference is compared to one ormore thresholds that provide an indication of interference between theone group and the other group. That is, an indicator indicatesinterference between the groups if the measured interferences have avalue that is above a threshold.

For an embodiment, the characterizing interference between at least onelink of a first group of the plurality of groups and at least one linkof a second group of the plurality of groups includes predictinginterference between links or groups of links based on modeling orsimulation of the wireless network. For an embodiment, thecharacterizing utilizes a topology of the wireless network. For anembodiment, the characterizing uses physical distance between links ornodes for characterizing or estimating interference between the links orgroups of links. At least some embodiments utilize previously determinedinformation of interference between links. Further, the characterizingmay utilize information of other parameters that influence propagationof interference signals, such as other data such, humidity etc.

For an embodiment, the characterizing interference between at least onelink of a first group of the plurality of groups and at least one linkof a second group of the plurality of groups includes a randomassignment of a code to a group of links. That is, the coding assignedto each group can be randomly assigned, whether or not any measurementsor predictions are made. Even assuming a random relationship of theinterference between the groups and assigning codes provides benefitsover not assigning the coding to groups of links.

A fourth step 1240 includes assigning at least one code of the subset ofcodes to the first group and at least one other code of the subset ofcodes to the second group based on the characterizing of theinterference. For an embodiment, a first code is assigned to the firstgroup and a second code is assigned to the second group. For anembodiment, the first code and the second code are identified orselected by an indicator of a level of correlation between the codes.

For at least some embodiments, a first sector associated with a firstspecific link that is within the first group configures packets fortransmission using the first code, and a second sector associated with asecond specific link that is within the second group configures packetsfor transmission using the second code. For an embodiment, configuringthe packets for transmission includes identifying an assigned code(reference sequence) based on that group that the specific wireless linkis within, and inserting the assigned code (reference sequence) into atleast a portion of the preamble. The sector then transmits theconfigured packet over the specific wireless link. That is, the firstsector inserts the first code into at least a portion of the preamble ofpackets for transmission over the first specific link, and the secondsector inserts the second code into at least a portion of the preambleof packets for transmission over the second specific link.

For an embodiment, the assignment of codes to groups of links isperformed while the wireless network is being designed or deployed. Thatis, the code selections, the grouping of the links of the wirelessnetwork, characterizing the interferences between groups, and the codeassignments are performed before deployment. These processes may beperformed, for example, by a network floor planning process.

For an embodiment, the assignment of codes to groups of links isperformed when the wireless network is modified. That is, at least oneof the code selections, the grouping of the links of the wirelessnetwork, characterizing the interferences between groups, or the codeassignments are performed as new sectors are added to the wirelessnetwork.

For an embodiment, the assignment of codes to groups of links isperformed periodically while the wireless network is in steady stateoperation. That is, at least one of the code selections, the grouping ofthe links of the wireless network, characterizing the interferencesbetween groups, or the code assignments are performed periodically whilethe wireless network is in steady state operation.

For an embodiment, the assignment of codes to groups of links isadaptively performed while the wireless network is in steady stateoperation. That is, at least one of the code selections, the grouping ofthe links of the wireless network, characterizing the interferencesbetween groups, or the code assignments are adaptively performed whilethe wireless network is in steady state operation. For example, ifperformance of the wireless network is sensed to be decreasing, thenetwork can adaptively repeat at least some of the steps of theassignment of codes to groups of links.

For an embodiment, the assignment of codes to groups of links isperformed manually. That is, at least one of the code selections, thegrouping of the links of the wireless network, characterizing theinterferences between groups, or the code assignments are triggeredbased on actions of a network operator.

The characterizing of the interference between the links of the firstgroup and the links of the second group provides for identification ofat least some interference between the links of the first group andlinks of the second group. Accordingly, codes are assigned to the firstgroup and the second group in order to mitigate interference (forexample, the early weak signal interference) of packets communicatedover one or more of links of the first group with packets communicatedover one or more of the links of the second group.

For at least some embodiments, the sectors include multiple antennaelements. Therefore, the sectors can form beam during transmission andreception of wireless communication signals. Due to the focusing andconcentration of beams, transmission and reception is improved, andself-interference within the wireless network is reduced as the energyof the wireless communication signals is focused in the beam formingdirection.

At least some embodiments include assigning different codes to eachgroup of pairs of groups of the plurality of groups based oninterference between at least one link of one group of a pair of groupsand at least one link of another group of the pair of groups. For anembodiment, different codes assigned to each group when interferencebetween groups is greater than a threshold.

For an embodiment, each group comprises a single link. That is, eachlink of the wireless network is identified as a separate group and eachlink is assigned a different code. In some wireless networks thatinclude many links, this may be onerous and processing intensive.

For an embodiment, a link is defined in part by a direction of theconnection between a transmitting sector and a receiving sector. Thatis, for an embodiment, one or more links of the wireless network areunidirectional. That is, each direction of wireless communicationbetween sectors of the wireless network may comprise an individual link.Accordingly, for an embodiment, wireless communication between twosectors in a first direction is a first link, and wireless communicationbetween the two sectors in a second direction is a second link. Further,the first link and the second link can be included within separategroups.

For an embodiment, each group includes one or more links. The number ofcode assignments can be reduced by grouping multiple links. Since a codeis assigned to each group, multiple links are assigned same code.

At least some embodiments include configuring one or more sectorsassociated with the one or more links of each of the plurality of groupswith the assigned at least one of the subset of codes. As previouslydescribed, each link forms a wireless connection between two sectors ofthe wireless network. Further, links of a group are assigned a code. Thesectors of each link code and decode packets communicated through thelink with the code (reference sequence) assigned to the link between thesectors. For an embodiment, the sectors are configured with the assignedcodes. For an embodiment, configuring the sectors includes providing theeach sector with a configuration parameter list, wherein theconfiguration parameter list includes the at least one of the subset ofcodes assigned to the grouped links of the node.

For an embodiment, each sector is assigned a single code for bothreception and transmission. This can simplify processing because thesector does not need to update the code the sector is using to codepacket or decode packets.

For an embodiment, each sector is assigned at least one code forreception and at least one code for transmission. For at least someembodiments, a node of the wireless network includes multiple sectors.If the sectors are communicating over a common channel, assigning onecode for reception and one code for transmission helps to minimizeinterference between sectors. Further, as previously described, at leastsome links are defined as unidirectional. Therefore, for at least someembodiments, different codes can be assigned to links that includecommunication in different directions. Accordingly, a sector may beassociated with one group when transmitting packets and with anothergroup when receiving packets.

Due to the grouping and code assignments, within a particular group, onesector may have an assigned code for transmission while another sector(e.g., a “reciprocal sector”) will have the same assigned code forreception. These assigned codes can stay the same, or change (if thelink directions are grouped differently) when the roles are reversed andone sector is receiving and the reciprocal sector is transmitting.

Code Selection

For a given system, there are multiple codes available for selection.For an embodiment, selecting the subset of the available codes includesselecting few enough codes to provide correlation between each of thecodes of the subset of codes of less than a threshold. That is, if toomany codes are selected for assignment, the correlation between any twoof the selected codes may not be great enough to allow a receiving nodeto properly distinguish between a desired packet and an interferingpacket.

Further, at least some embodiments include at least a minimum number ofcodes. For at least some embodiments, a code is assigned to each of theidentified groups. In order to minimize interference between the groups,a minimum number of possible codes for assignment may be desired.

At least some embodiments include selecting specific codes of the subsetof codes based on a capability or a characteristic of a receiver of atleast one sector of the nodes. That is, the sectors can includedifferent types of receivers that include different capabilities orcharacteristics. For an embodiment, the receiver type is identifiedduring ignition (for example, during deployment of the wirelessnetwork). For an embodiment, the receiver type is identified andcharacterized during design or initial simulation and testing. For atleast some embodiments, different receiver types react differently todifferent types of reference sequences. For an embodiment, differentreceiver types include different packet acquisition algorithms.

For at least some embodiments, the receiver characteristics which arepredicted, observed, or measured, are provided with the referencesequence which is selected based at least in part on the receivercharacteristics. For an exemplary embodiment, the reference sequencesare selected by feeding Golay sets into a standard (IEEE 802.11ad) Golaygenerator. The Golays sets in of Table 1 contain the sequence indices ofthe respective Golay sequences. The sequence is generated by using thebinary representation of the index as input (W matrix) to the standard11ad Golay generator. Note that index 5 represents the default Golayused in the 11ad standard.

TABLE 1 Golay Set Size = 2 {5, 69} Size = 3 {5, 59, 128} Size = 4 {5,59, 128, 66} Size = 5 {5, 59, 128, 66, 1}

As previously described, for at least some embodiments, grouping linksof the wireless network is based on connectivity of the links betweensectors of nodes of the wireless network is based upon a topology of thewireless network. For at least some embodiments, grouping links of thewireless network based on connectivity of the links between sectors ofnodes of the wireless network includes identifying one or more linksbetween sectors of the wireless network, and grouping the one or morelinks.

Characterizing Interference

For at least some embodiments, characterizing the interference includesgenerating an interference matrix wherein each entry of the interferencematrix includes at least one indicator of interference of an ith groupon a jth group. More specifically, for an embodiment, characterizing theinterference includes generating an interference matrix wherein eachentry of the interference matrix includes the at least one indicator ofinterference of an ith group on a jth group of the wireless network.Once created, the interference matrix provides a convenient vehicle foridentifying interferences between groups of links of the wirelessnetwork.

Alternatively, for at least some embodiments, characterizing theinterference includes generating an interference matrix wherein eachentry of the interference matrix includes at least one indicator ofinterference of an ith link on a jth link. More specifically, for anembodiment, characterizing the interference includes generating aninterference matrix wherein each entry of the interference matrixincludes the at least one indicator of interference of an ith link on ajth link of the wireless network. Once created, this interference matrixprovides a convenient vehicle for identifying interference between linksof the wireless network.

As previously described, for at least some embodiments, characterizinginterference between at least one link of a first group of the pluralityof groups and at least one link of a second group of the plurality ofgroups includes predicting interference of one or more links of thefirst group with one or more links of a second group based on simulationand/or testing of network pre-planning.

For at least some embodiments, characterizing interference between atleast one link of a first group of the plurality of groups and at leastone link of a second group of the plurality of groups includes sensinginterference of one or more links of the first group with one or morelinks of a second group. For an embodiment, interference between linksof groups is indicated if the sensed interference is greater than athreshold. For an embodiment, the interference between groups is sensedover time, allowing for adaptive updates to the code assignments. Thatis, over time different groups of links may cause interferences to besensed by other different groups. Accordingly, the code assignment forthe links can be updated as interference between the groups is sensed.

For at least some embodiments, characterizing interference between atleast one link of a first group of the plurality of groups and at leastone link of a second group of the plurality of groups includescharacterizing interference of a first number of links of the firstgroup with a second number of links of a second group. That is,interferences is indicated if the first number links of the first groupare determined to be interfering with the second number of links of thesecond group.

Code Assignment

As previously stated, for an embodiment, different groups are assigneddifferent codes. Further, for an embodiment, groups that interfere witheach other are assigned codes based on level of correlation between theavailable codes. That is, it is desirable to assign codes that are theleast (or below threshold) correlated to the groups that interfere witheach other.

For an embodiment, due to the finite number of available codes,different groups are assigned the same codes. That is, if there arelarge enough number of groups, all of the available codes can be used upduring assignment to the groups, and at least some codes will have to bereused with multiple groups. Ideally, groups assigned the same code donot have any interference with each other. However, if there is someinterference between groups an all available codes have been used, thecodes are reused based on the correlation between the available codesand the level of interference between the groups being assigned.

As will be described later, at least some embodiments further includereceiving, by a sector of the wireless network, data to be transmittedover a specific wireless link of the wireless network, wherein thespecific wireless link belongs to one of the first group or the secondgroup. A packet for transmission over the specific wireless link isconfigured, wherein the packet includes a preamble and the data. For anembodiment, configuring the packet includes obtaining a referencesequence based on the assigned at least one of the subset of codes ofthe first group or second group of the specific wireless link, andinserting the reference sequence into at least a portion of thepreamble. Finally, the configured packet is transmitted over thewireless link.

FIG. 13 is a block diagram of a wireless network, according to anembodiment. As shown the wireless network includes multiple nodes 1312,1314, 1316, 1318 and a central controller 1350. Further, one or more ofthe nodes 1312, 1314, 1316, 1318 include multiple sectors, such assectors 1110, 1111, 1112, 1113, 1114, 1115, 1117. As shown, the centralcontrol 1350 is connected to at least one of the nodes 1312, 1314.

For at least some embodiments, one or more of the steps of selecting asubset of codes from available codes, grouping links of a wirelessnetwork into a plurality of groups based on connectivity of the linksbetween sectors of the wireless network, characterizing interferencebetween at least one link of a first group of the plurality of groupsand at least one link of a second group of the plurality of groups, orassigning at least one code of the subset of codes to the first groupand at least one other code of the subset of codes to the second groupbased on the characterizing of the interference, occurs at the centralserver. For at least some embodiments, one or more of the steps ofselecting a subset of codes from available codes, grouping links of awireless network into a plurality of groups based on connectivity of thelinks between sectors of the wireless network, characterizinginterference between at least one link of a first group of the pluralityof groups and at least one link of a second group of the plurality ofgroups, or assigning at least one code of the subset of codes to thefirst group and at least one other code of the subset of codes to thesecond group based on the characterizing of the interference, occurs atone or more of the nodes or sectors of the wireless network. For atleast some embodiments, at least some of the steps occur upstream fromthe central controller, such as, in the cloud.

FIG. 14 is a flow chart that includes steps of a method of selectingcodes for sectors of a wireless network, according to an embodiment.That is, while the grouping was previously described as grouping oflinks, at least some embodiments include grouping of sectors. For thisembodiment, a first step 1410 includes selecting a subset of codes fromavailable codes. A second step 1420 includes grouping sectors of awireless network into a plurality of groups based on connectivitybetween sectors of the wireless network. A third step 1430 includescharacterizing interference between at least one interfering sector of afirst group of the plurality of groups and at least one receiving sectorof a second group of the plurality of groups. A fourth step 1440includes assigning at least one code of the subset of codes to the firstgroup and at least one other code of the subset of codes to the secondgroup based on the characterizing of the interference.

FIG. 15 is a flow chart that includes steps of a method of mitigatingpacket interference, according to an embodiment. A first step 1510includes receiving, by a sector of a wireless node, data to betransmitted over a specific wireless link of a wireless network. For atleast some embodiments, the wireless node includes a plurality ofsectors. Any given sector may receive data for transmission through aspecific link from another sector, or through a hardwire connection tothe wireless node. For an embodiment, a link is defined as a wirelesslink between a wireless transmitting sector and a wireless receivingsector.

For an embodiment, the specific link includes a wireless link betweentwo sectors of the wireless network. For an embodiment, a link (thespecific link) is unidirectional. For broadcast packets, for anembodiment, a link (the specific link) is defined by a transmitter andmultiple receivers.

A second step 1520 includes configuring a packet for transmission overthe specific wireless link, wherein the packet includes a preamble andthe data. For at least some embodiments, configuring the packet includesidentifying a reference sequence based on the specific wireless link,and inserting the reference sequence into at least a portion of thepreamble.

For at least some embodiments, identifying the reference sequenceincludes a central controller performing the selection of the referencessequence, and providing the reference sequence to the sectors. That is,for an embodiment, the central controller performs the identifying ofthe reference sequence, and the central controller provides thereference sequence to the wireless node. For an embodiment, thereference sequences are predetermined by the central controller or someother backend controller during network planning and/or deployment. Thereference sequence of a particular sector can be retrieved by thesector. For an embodiment, the sector performs the identifying of thereference sequence.

For an embodiment, the sector configures packets for multiple links, andinserts a reference sequence for each of the multiple links. For anembodiment, the reference sequence for a link (the specific link) canchange over time.

A third step 1530 includes transmitting, by the wireless node, theconfigured packet over the specific wireless link.

For at least some embodiments, the references sequence includes acomplex valued constituent base sequence. For an embodiment, the complexvalued constituent base sequence includes a complementary sequence. Foran embodiment, the complex valued constituent base sequence includes aWalsh code. For an embodiment, the complex valued constituent basesequence includes a pseudo random sequence. For an embodiment, thecomplex valued constituent base sequence includes a random complexsequence. For an embodiment, the complex valued constituent basesequence includes a Golay sequence.

FIGS. 16A, 16B, 16C show processes for inserting reference sequencesinto packets, according to embodiments. FIG. 16A shows the referencesequence being inserted into at least the preamble of the packet. Asshown, the packet includes an STF (short training field) and a CEF(channel estimate field). The reference sequence is inserted into atleast a portion of these fields. For an embodiment, the referencesequence is pre-appended into the at least the portion of the preamble.The pre-appended reference sequence allows for a receiver to properlylock onto the packet earlier in the duration of the packet.

FIG. 16B shows a repeating of the insertion of the reference sequencewithin the preamble. The reference is designated with a G (Golay code).Further, for an embodiment, a phase of the reference sequence changesduring the repetition of the reference sequence. For example, as shown,the phase alternates from Golay code to Golay code as indicated by the“1” and “−1”. The alternating phase of the sequence as shown is merelyan example. Other repeating sequences can alternatively be utilized.

FIG. 16C shows additional insertion of the reference sequence into thepacket. For an embodiment, reference sequences are additionally insertedinto data of a payload of the packets. Further, for an embodiment, thereferences sequence is additionally inserted into a post-able 1090 ofthe packet.

For at least some embodiments, identifying a reference sequence based onthe specific wireless link includes characterizing interference betweenthe specific link and at least one other link of the wireless network,and assigning the reference sequence to the specific link, and anothersequence to the at least one other link of the wireless network. For atleast some embodiments, identifying a reference sequence based on thespecific wireless link, includes selecting a subset of codes fromavailable codes, grouping links of the wireless network into a pluralityof groups based on connectivity of the links between sectors of nodes ofthe wireless network, characterizing interference between at least onelink of a first group of the plurality of groups and at least one linkof a second group of the plurality of groups, wherein at least one ofthe first group or the second group includes the specific wireless link,assigning at least one code of the subset of codes to the first groupand at least one other code of the subset of codes to the second groupbased on the characterizing of the interference, and configuring thesector with the references sequence, wherein the reference sequencecomprises one of the subset of codes of the first group or one of thesubset of codes of the second group based on which of the first group orthe second group includes the specific wireless link.

FIG. 17 is a flow chart that includes steps of a method of receiving apacket in which a reference sequence is inserted in a preamble of thepacket, according to an embodiment. A first step 1710 includesreceiving, by a sector, a packet over a specific wireless link, whereinthe packet includes a preamble, and the data. A second step 1720includes identifying a reference sequence based on the specific wirelesslink. A third step 1730 includes correlating at least a portion of thepreamble with the reference sequence that is selected based on thewireless link. A fourth step 1740 includes receiving, by the wirelessnode, the configured packet over the specific wireless link.

FIG. 18 shows test results of packet interference within a wirelessnetwork for different codes selections, according to an embodiment. Afirst table 1810 shows the number of early weak interferers for a samplenetwork that has a total number of 3260 links and includes standard 801ad assignments. As depicted, the number of links with one early weakinterferer is 134, the number of links with two early weak interferersis 62, and the number link with three early weak interferers is 1.

A second table 1820 shows the number of early weak interferers for thesample network that has a total number of 3260 links and includes linkbased and node based assignments using two Golay codes as referencesequences. As depicted, for the link-based assignments, the number oflinks with one early weak interferer is 65, the number of links with twoearly weak interferers is 1, and the number link with three early weakinterferers is 0. As depicted, for the node-based assignments, thenumber of links with one early weak interferer is 66, the number oflinks with two early weak interferers is 4, and the number link withthree early weak interferers is 0.

A third table 1830 shows the number of early weak interferers for thesample network that has a total number of 3260 links and includes linkbased and node based assignments using three Golay codes as referencesequences.

A fourth table 1840 shows the number of early weak interferers for thesample network that has a total number of 3260 links and includes linkbased and node based assignments using four Golay codes as referencesequences.

FIG. 19 shows a receiving node that includes received signal gaincontrol, according to an embodiment. For an embodiment, the receivingnode is operative to facilitate a network-dependent gain control ofsignals received by the receiving node. At least some embodimentsinclude utilizing gain control of the receiving node to mitigate theeffects of early packet interference. FIG. 19 shows an RF (radiofrequency) chain 1910 of a receiving node. As shown, the RF chain 1910includes at least a GCM (gain control module) 1911 and a frequencyconverter 1912.

As shown, the receiving node further includes a baseband 1920. For atleast some embodiments, the baseband 1920 includes at least a GCM 1921and an ADC (analog to digital converter) 1922.

For an embodiment, the GCM control adaptively adjusts a gain of receivedsignals to provide the baseband processing with a signal for processingwith a semi-consistent amplitude signal. For at least some embodiments,the GCM selection is adjusted to a smaller gain with signals receivedwith high signal power, and the GCM selection is adjusted to a largergain for signals received with low signal power.

FIG. 20 is a plot that shows GCM selections over time, according to anembodiment. High-levels of gain control indicate a received signalhaving a low signal amplitude or a low-power level, and low-levels ofgain control indicate a received signal have large signal amplitude orhigh-power level.

For an embodiment, it is assumed that low-power received signals aremore likely to be interference and high-power received signals are morelikely to be the desired signals. Accordingly, monitoring the GCM in afree-running (gain not externally controlled) state can be used toselect a set GCM level. For example, the GCM can be monitored over timeto generate selected values of gain control over time, such as shown inFIG. 20. By selecting the GCM to be set (to, for example, the selectedGCM 2020 of FIG. 20) the reception by the receiving node of aninterfering signal or packet can be reduced. That is, monitoring of theselected GCM levels as depicted by the curve 2010 of FIG. 20 can be usedto identify the timing of the reception of interfering packets asrepresented by the higher GCM gain selections. The GCM of the receivingnode can be set to a selected GCM gain setting to reduce the receptionby the receiving node of interfering signals because the GCM gain isselectively set too low for the interfering packets or signals to bereceived by the receiving node. For an embodiment, the GCM of thereceiving node is adjustable, but the selected GCM gain is a maximumlevel of gain provided, That is, that selected GCM gain is a maximumgain setting of the GCM.

FIG. 19 shows possible GCM control (GCM control1, GCM control2) ineither a GCM 1911 of the RF chain 1910 and/or GCM 1921 of the baseband1920.

For an embodiment, a GCM tracking and statistical analysis block 1932 ofFIG. 19 tracks and monitors the free-running GCM, and then selectscontrol of the GCM (1931) based on the tracking and statistical analysis[[for a given period of time, either periodic or triggered, send a lotof packets, and for each packet, determine base gain. Plot frequencydistribution and choose something at the lower end of the frequencydistribution]] of the free-running GCM. For an embodiment, the trackingand statistical analysis includes periodically or adaptively (triggered)determining a base (GCM) gain that are selected for a plurality ofreceived packets. A frequency distribution of the selected GCM gains isanalyzed and a GCM gain is selected (as either a fixed gain or a maximumgain) is selected at a lower end (less than a threshold) of thefrequency distribution. For at least some embodiments, an externalcentral/cloud controller 1940 performs at least some of the monitoring,statistical analysis, and selection of the GCM control.

Further, for at least some embodiments, the GCM control 1931 isadditionally or alternatively controlled based on scheduling of thetransmission and reception of packets (1933) between transmitting andreceiving nodes. For example, based on scheduling of packet transmissionprovided by a MAC (media access control) of the external central/cloudcontroller 1940, the receiving nodes can selectively reduce the GCM tomitigate the reception of interfering packets. That is, the schedulingprovides timing of when desired packets are to be received by thereceiving node. The receiving node can reduce the GCM before the timesin which desired packets are to be received, thereby reducing thepossibility of inadvertently receiving interfering packets. The GCM isthen increased for reception of the desired packets, wherein the timingof the GCM increase is based on the scheduling of the desired packets.

Further, for at least some embodiments, the external central/cloudcontroller 1940 further provides scheduling (for example, MACscheduling) of the transmission of packets by the interfering node.Accordingly, the external central/cloud controller 1940 can indicate tothe receiving node the timing in which the interfering node is totransmit potentially interfering packets. Accordingly, the GCM can bereduced based on the timing of the scheduled transmission of potentiallyinterfering packets, thereby reducing the chances the receiving nodewith receive the interfering packets.

FIG. 21 shows a transmitting node 2110, a receiving node 2120, aninterfering node 2140, and a central controller 2130, according to anembodiment. For at least some embodiments, the central controllerreceives various network parameters, and adjusts the gain of the GCM ofthe receiving node to mitigate the reception of interfering packets. Foran embodiment, the central controller 2130 receives NW (network) mappinginformation that allows for a determination of interfering nodes. For anembodiment, the central controller 2130 receives NW topology informationthat allows for a determination of interfering nodes.

For an embodiment, the central controller 2130 controls the transmissionof training signal between the transmitting node 2110 and the receivingnode 2120. Further, for an embodiment, the central controller 2130controls the transmission of training signal between the interferingnode 2140 and the receiving node 2120, thereby allowing acharacterization of interference between the interfering node 2140 andthe receiving node 2120.

For at least some embodiments, the central controller 2130 uses anycombination of the NW mapping information, the NW topology information,training information of the transmission channel between thetransmitting node 2110 and the receiving node 2120, or traininginformation of the transmission channel between the interfering node2140 and the receiving node 2120.

For an embodiment, the central controller 2130 provides scheduling (forexample, MAC (media access control)) for the transmission of packets bythe transmitting node 2110 and reception of packets by the receivingnode 2120. For an embodiment, the receiving node 2120 reduces the GCMbefore the scheduled timing of the reception of the desired packets inorder to reduce the possibility of reception of interfering packets.Further, the central controller 2130 provides an indicator to thereceiving node of when the central controller 2130 has scheduled theinterfering node 2140 to transmit potentially interfering packets.Accordingly, the receiving node reduces the GCM during the scheduledtiming of the potentially interfering packets to reduce the possibilityof the receiving node inadvertently receiving the interfering packets atthe expense of reception of the desired packets. As stated, for anembodiment, the central controller 2130 provides scheduling of packettransmission of the transmitting node, the receiving node, andinterfering nodes. Further, the central controller 2130 can aid in thedetermination and characterization of interference between the nodes.That is, the central controller 2130 can monitor interference betweennodes and aid in the identification of which nodes interfere with othernodes. Based on the interference determinations, the central controller2130 can provide GCM control as described above to mitigate the effectsof packet interference.

Embodiments of the subject matter and the operations described in thisspecification can be implemented in digital electronic circuitry, or incomputer software, firmware, or hardware, including the structuresdisclosed in this specification and their structural equivalents, or incombinations of one or more of them. Embodiments of the subject matterdescribed in this specification can be implemented as one or morecomputer programs, i.e., one or more modules of computer programinstructions, encoded on a computer storage medium for execution by, orto control the operation of, a data processing apparatus.

A computer storage medium can be, or can be included in, acomputer-readable storage device, a computer-readable storage substrate,a random or serial access memory array or device, or a combination ofone or more of them. Moreover, while a computer storage medium is not apropagated signal, a computer storage medium can be a source ordestination of computer program instructions encoded in an artificiallygenerated propagated signal. The computer storage medium also can be, orcan be included in, one or more separate physical components or media(e.g., multiple CDs, disks, or other storage devices). The operationsdescribed in this specification can be implemented as operationsperformed by a data processing apparatus on data stored on one or morecomputer-readable storage devices or received from other sources.

The term “processor” encompasses all kinds of apparatus, devices, andmachines for processing data, including, for example, a programmableprocessor, a computer, a system on a chip, or multiple ones, orcombinations, of the foregoing. The apparatus can includespecial-purpose logic circuitry, e.g., an FPGA (field programmable gatearray) or an ASIC (application-specific integrated circuit). Theapparatus can also include, in addition to hardware, code that createsan execution environment for the computer program in question, e.g.,code that constitutes processor firmware, a protocol stack, a databasemanagement system, an operating system, a cross-platform runtimeenvironment, a virtual machine, or a combination of one or more of them.The apparatus and execution environment can realize various differentcomputing model infrastructures, such as web services and distributedcomputing and grid computing infrastructures.

A computer program (also known as a program, software, softwareapplication, script, or code) can be written in any form of programminglanguage, including compiled or interpreted languages and declarative orprocedural languages, and it can be deployed in any form, including as astand-alone program or as a module, component, subroutine, object, orother unit suitable for use in a computing environment. A computerprogram may, but need not, correspond to a file in a file system. Aprogram can be stored in a portion of a file that holds other programsor data (e.g., one or more scripts stored in a markup languagedocument), in a single file dedicated to the program in question, or inmultiple coordinated files (e.g., files that store one or more modules,subprograms, or portions of code). A computer program can be deployed tobe executed on one computer or on multiple computers that are located atone site or distributed across multiple sites and interconnected by acommunication network.

The processes and logic flows described in this specification can beperformed by one or more programmable processors executing one or morecomputer programs to perform actions by operating on input data andgenerating output. The processes and logic flows can also be performedby, and apparatus can also be implemented as, special-purpose logiccircuitry, e.g., an FPGA or an ASIC.

Processors suitable for the execution of a computer program include, forexample, both general and special-purpose microprocessors and any one ormore processors of any kind of digital computer. Generally, a processorwill receive instructions and data from a read-only memory or a randomaccess memory or both. The essential elements of a computer are aprocessor for performing actions in accordance with instructions and oneor more memory devices for storing instructions and data. Generally, acomputer will also include, or be operatively coupled to receive datafrom or transfer data to, or both, one or more mass storage devices forstoring data, e.g., magnetic disks, magneto-optical disks, or opticaldisks. Devices suitable for storing computer program instructions anddata include all forms of nonvolatile memory, media and memory devices,including, for example, semiconductor memory devices, e.g., EPROM,EEPROM, and flash memory devices; magnetic disks, e.g., internal harddisks or removable disks; magneto-optical disks; and CD-ROM and DVD-ROMdisks. The processor and the memory can be supplemented by, orincorporated in, special-purpose logic circuitry.

From the foregoing, it will be appreciated that specific embodiments ofthe invention have been described herein for purposes of illustration,but that various modifications may be made without deviating from thescope of the invention. Accordingly, the invention is not limited exceptas by the appended claims.

We claim:
 1. A method for use in a wireless network, comprising:constructing, by a sector of a transmitting node, a packet includingdata that is to be transmitted to a receiving node in the wirelessnetwork, wherein the constructed packet includes a short training field,a channel estimation field, a header field, and a data payload; whereinthe constructed packet is to be transmitted to the receiving node over aspecific link between the sector and the receiving node, and whereinconstructing the packet for transmission over the specific wireless linkcomprises: identifying a reference sequence based on the specificwireless link, wherein identifying the reference sequence comprises:selecting a subset of codes from available codes; grouping links of thewireless network into a plurality of groups based on connectivity of thelinks between sectors of nodes of the wireless network; characterizinginterference between at least one link of a first group of the pluralityof groups and at least one link of a second group of the plurality ofgroups, wherein at least one of the first group or the second groupincludes the specific wireless link; assigning at least one code of thesubset of codes to the first group and at least one other code of thesubset of codes to the second group based on the characterizing of theinterference; and configuring the sector with the reference sequence,wherein the reference sequence comprises one of the subset of codes ofthe first group or one of the subset of codes of the second group basedon which of the first group or the second group includes the specificwireless link; and transmitting, by the sector of the transmitting node,the reference sequence before the short training field of theconstructed packet, thereby reducing a likelihood that the receivingnode will decode a different short training field of an interferingpacket before the receiving node decodes the short training field of theconstructed packet.
 2. The method of claim 1, further comprising codingthe short training field of the constructed packet to uniquely addressthe receiving node.
 3. The method of claim 1, further comprising codingthe header field of the constructed packet to uniquely address thereceiving node.
 4. The method of claim 1, wherein the sector performsthe identifying of the reference sequence.
 5. The method of claim 1,wherein a central controller performs the identifying of the referencesequence, and the central controller provides the reference sequence tothe sector.
 6. The method of claim 1, wherein inserting the referencesequence into the at least the portion of the preamble comprisespre-appending the reference sequence to the at least the portion of thepreamble.
 7. The method of claim 1, wherein the reference sequenceincludes a complementary sequence.
 8. The method of claim 7, wherein thecomplementary sequence includes a Golay sequence.
 9. The method of claim1, further comprising repeating the reference sequence inserted into thepreamble.
 10. The method of claim 9, wherein a phase of at least aportion of the repeating reference sequence changes within the preamble.11. A transmitting node of a wireless network, comprising: atransceiver; an antenna; a processor; and a memory for storing programinstructions that are executable by the processor to: construct a packetincluding data that is to be transmitted to a receiving node in thewireless network, wherein the constructed packet includes a shorttraining field, a channel estimation field, a header field and a datapayload, wherein the constructed packet is to be transmitted to thereceiving node over a specific link between the sector and the receivingnode, and wherein constructing the packet for transmission over thespecific wireless link comprises the program instruction executing theprocessor to: retrieve a reference sequence, wherein retrieving thereference sequence comprises: selecting a subset of codes from availablecodes; grouping links of the wireless network into a plurality of groupsbased on connectivity of the links between transmitting nodes of nodesof the wireless network; characterizing interference between at leastone link of a first group of the plurality of groups and at least onelink of a second group of the plurality of groups, wherein at least oneof the first group or the second group includes the specific wirelesslink; and assigning at least one code of the subset of codes to thefirst group and at least one other code of the subset of codes to thesecond group based on the characterizing of the interference; andconfiguring the transmitting node with the reference sequence, whereinthe reference sequence comprises one of the subset of codes of the firstgroup or one of the subset of codes of the second group based on whichof the first group or the second group includes the specific wirelesslink; and control transmission by the transceiver through the antennathe reference sequence before the short training field of theconstructed packet, thereby reducing a likelihood that the receivingnode will decode a different short training field of an interferingpacket before the receiving node decodes the short training field of theconstructed packet.
 12. The transmitting node of claim 11, wherein thereference sequence includes a complementary sequence.